Electronic
Fuel
Injection
(EFI) Troubleshooting
(ECU)
SideKick & Trackers. 89-98The purpose of the EFI system (spark and
injection) is to recover the
lost 25% efficiency of Carburettors and to keep the air clean
(keeps the CAT running, as much as possible)
Some studies show 35% efficiency over carbs.!My page does not apply to hacked or
modified
cars. Stock cars, not with CAI mods, or any other gross EFI
modifications, or removed CATS or EGR.
There are scan tools
that allow you to see live data and how most things are working or
not.
If you have 1996+ USA car, all scan tools made support this
Sidekick
The EFI system is totally dependent on inputs. (sensors)
The #1 input ,is called a constant; the ECU expects 30PSI fuel pressure
running, if not near there, the ECU may fail to inject properly, the factory spec is clear on this.
The Injectors must work and not leak and the spark must be hot
(blue/white) and timed correctly.

In many cases, the motor will not hold closed loop . ( this is why the
gas mileage is bad)Closed Loop is goal
at idle and
light cruising, and never happens wide open
throttle (WOT) or decelerating (cut fuel) or flying up tall
hills.
Close loop drops out normally, if you move the throttle pedal fast ,
then after
you steady your foot, close loop
returns. (if not, its busted)

Here is a list of steps to take. if this happens to you. including the
PRO shop action and DIY action (Do It
your self or
shade-tree action)
The First Problem or Compliant :
Motor runs rich, it stinks of raw unburned fuel at the tail pipe or
even
black smoke emitting from the same.
My Spark plug tips keep fouling (black
carbon, wet or both).
Or the motor runs lean, bogs, and can even stalls.

Most likely the motor (ECU) is in Limphome mode or
O2 sensor
is stuck
measuring LEAN for ever. But motor is rich.
The exhaust is cracked near the front 02 sensor render it useless.
(full or part of the time)
Most people ignore the horrible MPG because running rich makes plenty
of power or don't notice the Limphome mode symptoms. (lost power)
The ECU will throw DTC
errors if it has bad Inputs.

This ECU can not (mostly) report sensors or actuators that are no
longer accurate
, it can only report DEAD ONES, and only a few.
The newer ECU 96+ do report , better, EGR tests, lean , too rich,
and others, see "monitors" under OBD2 tech.
This old system has little redundancy and that is the core design flaw
of most EFI systems.
The ECU can only believe a sensor is good, unless its dead . It
believes the sensors with blind trust.

The ECU runs in 3 states (besides cranking, cold start,
warm up ,
hot open loop and closed loop).

Sensors good in Closed
loop
( when I say sensors
here, I mean all inputs and actuators and fuel pressure) For Maximum
MPG
and clean Air.

Sensors way out of
calibration (offset,biased,
inaccurate) but the ECU knows something is very wrong but can
figure out what ! Limphome Happens

There is a dead sensor mimic mode but I consider
that just a
feature of Limphome. Using the mimic mode one can drive home
,
with MAF pulled ( a trick with a weak MAF)
And a "BACKUP mode" ( ECU processor is Dead !)
there are no
documents for that mode , anywhere but is real. It is BAD ECU detected,
now in back up (rich and low power)
Hint:
Most N.American Sidekicks with the CAT
missing (zero
back pressure) will cause the EGR to fail ,when that fails (code 51)
then the EFI will go to limphome. 91+ years.

CAT Missing? melted inside, rodded out, test piped, blowout, or just
somehow not there.Givens: (preconditions)
There must be no air leaks in the exhaust system forward of
the
CAT (converter) . No cracks, no gaskets bad or missing! Remember;
Exhaust blows and sucks !!!
There must not be any illegal induction leaks (air leaking to this
vacuum or up stream of the MAF , from the MAF to the intake valves).
{Un-metered Air}
The CEL lamp is off running and you have corrected all DTC errors
logged. (DTC
=
Diagnostic trouble codes)
Hot motor , idle set to 800 RPM with top of TB bleed screwsall
accessories off.
Water (coolant) at 180 °F {82C} or higher ( use a real meter to
measure this, DO
NOT guess and a IR thermo gun
is best.)
Working with no tools is quite impossible. The tools are
free, think about that? I will try my best to avoid
tools,
for this page.
If the MPI injectors
are bad,
the EFI system will never work right or
make any sense. The ECU may hunt and go quite
crazy. {EFI
weakness #1}
The same with bad fuel pressure. {EFI weakness #2} Low fuel pressure
is so
common and it seems no one ever checks it.
The EGR main valve must not Leak (valve blocked open by carbon, or
fresh air leaking past its gasket)
The Catalyst exhaust converter (CAT) must not be melted. Check exhaust back
pressure.
The motors
sensors must be
operation.
The Engine must be viable , with good compression and steady vacuum (no
pulsing vacuum gauge needles allowed) The engine must pump air
perfectly !
A pro starts with the Gas Analyzer at the exhaust tip but a DIY person
,
works with his noses ,eyes and a DMM.

We are now connected to the FRONT O2
sensor, with either a DMM or DSO (meter or scope).
(requires back probe leads and back probing techniques)

This sensor can do 3 things, show rich full time ,
lean
full time or it will swing (proving normal closed
loop
operation)

The ECU trust the 02 100 % , at idle and steady light load
cruise. if the sensor is biased(bad) the EFI will inject poorly under
these conditions.

If the O2 sensor reads rich then it is reporting the
true
facts, we can lean the mixtures artificially by opening up
the
plenum (induction manifold) by causing a leak , and one place to
do that are the nipples and hoses (1/4") connected to
it. I
use the EVAP nipple. This will send illegal air (unmetered on
,16v only) to the engine, add more leaks as needed and the O2
sensor must start lowering its high voltage 0.9v
(900mV) and reducing shows that not only does the O2 function
correctly but
that it can SEE this leaner condition occur, you might even
trip
off the CLOSED loop mode and the 02 might starting
swinging from .1v to .9v.

If you can not reduce the rich reading O2 in step 8 ,
the
O2 is either
bad (MAF offset too high) or the fuel pressure is way too high ,
measure the fuel pressure
now. See
my pump pages
for spec and ways.

Rich always, can be caused by ECT stuck at 10,000
ohms
instead of 300 HoT. Missing/bad
thermostat or stuck open or the
wrong one. (common on 15year old cars).

Rich/ lean can be caused by a bad MAF (stuck high or
reading more
than 20% higher than reality will cause very rich condition)

Now we pretend that the 02 is reading lean, but motor
is
running rich. ! (there are 4 states considering 02
agrees
with reality or not)

Try to add propane gas to the suction side of the air
cleaner (filter off , idling) This illegal (not
injected)
fuel will make the motor go richer and the 02 sensor must see
this and make the Lean 02 reading rise in voltage, if not
then
the 02 is bad or you have a massive AIR to exhaust leak. Exhaust leaks are never tolerable by the O2
sensor.

I will now reverse the rulesand
symptoms, here for the motor running lean.
(actually is
lean running)

IF the motor is lean ,then the motor will want to
bog. (a bog equals
long
hesitations,or
loss of power or
back firing into the induction (hiccups))

This can be caused by low fuel pressure (measure it ,
see
pump
link above line 9) you can measure the 02 sensor and it will read lean
(low and stuck)

Or be caused by a dead/weak MAF (stuck lower low by
20% of
reality) or
a massive induction air leak. (induction leaks bypass the MAF and
defeats its whole purpose)

Again add propane gas (raw) to the air
cleaner
suction and see if you can move the readings. or even bring ON CLOSED
LOOP (CL)

Keep in mind if you have an air leak in the plenum
(induction) and you add propane, the motor will instantly
race.
(lean + fuel = race). Keep RPM in mind here.

If you can not raise the lean running 02 reading then
the
02 is bad or you have a massive air link in the INTAKE.

If you had a massive air leak in the exhaust, near the
02
this front sensor will go NUTS. (lean 02 and huge EFI rich, or will
cycle like mad, lean then rich)

This can be caused by low MAF, induction leaks ,low
fuel
pressure and or bad fuel pressure regulator or a weak fuel
pump
or low power feed to said pump.

All statements on the page assumes you have checked to see that your
motor pumps air. (it's really not an engine ,it's an air pump
to
all ASE mechanics)
Do a Compression test, and vacuum test. Do it now or you will
chase you tail
,just like a puppy.
Look for:
170 PSI minimum all 4 cylinders with wide open throttle (WOT)
idle 800 RPM , hot, 19 inches HG vacuum. the needle
the
gauge is NOT wigging at all. Steady as rock , hands and foot off that
throttle ok?
if not ?,then the motor is bad.
Bad motor means , engine pumps air perfect (no stuck open EGR) no
melted CAT and perfect steady vacuum at idle.
Not checking fuel pressure KEYON is a real bad idea. This is the
1st check after checking , bad motor, bad spark.
The ECU assume fuel pressure is at spec. at all times. There is NO fuel
pressure monitor or sensor. The ECU can not warn you , in most cases.
(96 and later , maybe)
Checking all sensors and actuators for connections and
damaged
wiring now, is the first step. INSPECT all ECU and EFI
wiring for damage.
Missing sensors. and OEM parts.
Hacked wiring in the EFI system is a no GO.
Missing
EGR or (leaking any any way shape or forum that includes a bad EGR base
gasket) EGR mains , love to stick open as you come to a stop sign .
A Missing Cat con ,kills the EGR dead. (the live test of the EGR
will prove that point)

This information above will allow you to hone in on the causes of rich
or lean motor.
The 8 valve motor behaves quite different with similar malfunctions
,which I will cover next.

Any breach (leak) from the Throttle body air horn to the
MAF all cause lean. (The ECU can compensate for tiny leaks,seen as LTFT
showing add fuel)

Any PCV hose , or IAT sensor air hose cracked, will cause
lean.

A dirty MAF causes lean. (lost power) especially W.0.T.

There must not be any air leaks
(abnormal) from the engine intake
valves, all the way too the MAF.

All air leaks show up worse at idle.
The ECU can not compensate for leaks, when running in normal open loop
mode, at all, because the 02 is not used then.
Accelerating is one such time. and off idle, tip-in acceleration can be
real bad, with air leaks.

The same things happen as per above but on this motor any induction air
leaks Naturally cause
a high idle and no loss in
power or CL status.
That is correct.
This motor eats air leaks for breakfast, it acts just the
same,
as stepping on the gas (air) pedal.

So adding extra air will only raise RPM and never change O2 readings.
(on a working 8v system !) Induction air leaks on this system cause
high RPM with full power.
In fact do that, pull the EVAP nipple and see the RPM
rise.
(if it don't rise , you are ON TRACK , the MAP is dead or fuel pressure
is low)
If it does not ,then the fuel pressure is very low or the MAP is dead.
The same tests on the 02 sensor are as follows:
8V top fails: (Rich)

stuck in limphome (lots of reasons)

Fuel pump regulator FPR, is stuck closed fuel
pressure on the
moon, even as high is 60PSI (BAD) 30 PSI is normal.

Fuel return line pinched (tank to pf reg output)
60PSI. Horrid MPG.

FPR stuck open , causing low fuel pressure, a
hopeless case.

Exhaust leak (slams O2 to lean for ever) Motor to
rich as
RESULT. horrid MPG. (or cycles, from gross lean to gross rich over and
over)

Lean: (bogging)
Bad MAP, fuel pressure low, Bad O2. ( most exhaust
AIR
leaks cause a rich motor but not not all , due to a design flaw in all
02 sensors)
O2 sensors can read wrong due to the carbon blanket effect , Bosch has
white papers all about it, just imagine covering the sensor with your
palm, and now the o2 is blind to the presents of
oxygen.
The O2 will signal rich. (falsely and is not bad) This is the O2
sensors
greatest weakness. After correcting bad (rich) AFR
, the O2
will clean
itself and once again operate.
Bad MAFs are very rare. as is, weak or out of calibration MAPs)
Fuel pressure what is the fuel pressure, and what is the voltage to the
fuel pump , only 8vdc? bingo the problem.
O2 sensor or bad MAF/MAF .
or Illegal induction air leaks. (16v)

Rich: (stinking and low MPG)
Bad / weak MAP, high fuel pressure (out of band) , Bad 02
sensor or an Air leak in the exhaust forward of the center
line
of the CAT converter.
Leaking Injectors (inside the valve is stuck open) or the
seals
on the outside leak (look at it idling, it is viable)

If the MAP error is over 20% it is bad .(note it has a range
of
valid outputs for a
given
vacuum input)

Bottom line. If you lie to the ECU bad enough the 02 sensor can not
save you. If the O2 sensor is bad, there
is no hope
at all.

CLOSED
LOOP fuel
trim. The old GM cars work like
(similar ) to our OLD ECU cars pre 96 years
The 1996+ and newer cars use OBD2 and make these checks
real easy .

Keep in mind all 02 sensors are designed to only
operate at
Stoich. 14.7:1 Air fuel ratio or AFR for short ( or
chemically perfect
combustion
!)
Stoich is short for stoichiometric, the
chemical perfect
burning of the fuel using the exact amount of air. Think
Perfect
burn or best MPG (close).

To understand the Closed loop and how the ECU controls
Air
Fuel mixtures
one has to read about block learn and fuel trim.
There are 2 variables in the ECU that store SHORT term fuel trim (STFT
,aka : integrator) and long term fuel trim (aka : block learn mode).Integrator and BLM are
only GM terms. So we will try to avoid them. and
use the new standard SAE terms.
The STFT changes many times a second and LTFT changes every second or
two. These terms are SAE.org terms, a standard of
the
industry.
See these pages , from my favorite authors.
The first is by Jeff Hartman, a famous motor books author (
the
best , in my opinion)http://www.chevyhiperformance.com/techarticles/49738_fuel_injection_basics/index.html

My rule, is to not trust any old 02
sensors, at all , I just put in a fresh one for $20 and now I can trust
the swinging 02 and all trim readings.

The ECU has up to 16 PID's that are the storage values for LTFT.
LTFT is long term fuel trim, it stores the Fuel correction
values, for many running conditions of the engine. (idle is 1)
This EFI works the same way !

The LTFT is the coarse adjust for fuel trim. Slow,
every second. (not so slow huh?)
The
STFT is the vernier adjust for fuel trim. Fast. (many
times a second) I ignore this, because its too fast to watch.
If the STFT rails (128 is the center) to 0 or 255 ( 8 bit
storage
word in ECU memory) 1 byte to geeks.
Both values are 1 byte and are a major part of the AFR total
equation.

The total equation uses tables, ECT, MAF/MAP , RPM, Load, as other
factors for AFR. But the O2 is king of the mountain and rules the roost.
128 means , I (ECU) am not adding or subtracting fuel, I am using fuel
tables and they are working perfectly now. (O2 says so. and I believe
it says ECU) When the Integrator
STFT,
is less than 128, the computer is subtracting fuel from the base
computation.
The 0BD2 car shows zero ,0, for perfect AFR, that is , its not
correcting fuel trim at all. (eg. the MAF is doing a perfect job
on air flow)

These rules are only OBD1 rules "128" etc, OBD2 reports
Percentages using any scan tool.
My OBD2 car runs about -3% at idle hot. (a very good reading)

Open loop is when the ECU uses the Total AFR equation minus the 02
readings, and in which case STFT and LTFT in Not applicable.
The ECU drops the usage of the 02 , when not at closed loop. (eg. WOT.
fast acceleration, up tall hills or at deceleration mode, foot
off gas pedal in gear coasting"cut fuel")
Any car with bad fuel mileage , you may find the LTFT dead, or stuck or
reading badly for the driving conditions.

The o2 sensors may be stuck lean, and AFR is really very rich. LTFT in
this case would show minus huge numbers. or near -100 (OBD1) or -34% OBD2
On new cars they limited the extremes of LTFT , (and DTC'S when HIT) on
OBD2 cars.
The new rules are Long +Short > +43% or lower than -34% (or
ether over 20%)
or STFT short below -19% or long trim alone below -16% (exhaust
leak near 02 or 02 bad? or full pressure on the moon (60psi?)
DTC 171 or 172 are the warnings for above #

My example:
Lets pretend the 02 sensor (aka: Oxygen Sensor) shows lean
and
the STFT went to 255. ( it will do this very very fast, 1 second or
less)
This happens because the ECU keeps adding fuel (guessing) and
the
STFT rails (in my example) and the AFR is still too lean. (the o2
sensor says so)
If the STFT rails,then the ECU then moved the fuel to 255 rich and the
motor was still too lean, so it sets the LTFT to 129. and resets the
STFT to 128.
The hunt then again starts over looking for Stoich (14.7:1)
or
simply looks for the 02 sensor to switch. It really is just a switch
(Narrow band sensors).
Then the ECU hunts for Stoich some more and repeats this guessing,
until it finds the crossover point. ( the O2 sensor, toggled
rich for the first time)
That point where the 02 reading finally shows the cross from lean to
now rich.
The ECU now tries to toggle back and forth from left and right of
Stoich, but just a tiny amount (a secret amount).
This looping is what makes the 02 swing and cross counts. over and over
never ending. (look with any scope to see what the 02 is
doing?)Things go wrong:
If lets say the ECU keeps railing (255) LEAN, the STFT then the LTFT
will keep climbing (adding fuel) at some point depending on
year
of car. the LTFT will get near the RAIL
and hit a threshold ( lets say
220) [I'm guessing ,
because the actual value, is a secret] but if the 220 LTFT is
hit
, some ECUs (after 1996) will throw a fuel trim error DTC.
On older cars,( pre 96) it may just drop into limphome, for a
time or until car is restarted. ( big secret 2)
On older yet,cars 89? There may be no threshold at all (ouch)
,
an the LTFT rails and stays there.
Rich
as a pig , most likely.
If the 02 sensor was lying all the long time. then the stupid ECU rails
and runs rich all the time. <<
good
reason to buy and keep a fresh O2 sensor (front) in your car.
There is more to this Algorithm then my simple example( eg:
ECU
purposefully goes rich and lean extra amounts to keep Catalyst hot and
active(needs fuel to do that and oxygen).
The ECU purposefully wastes fuel to keep the CAT running. but
the
average AFR is "STOICH".
But forget that, all we care? , is can I hold closed loop and is that
loop centered around Stoich.?

If the 02 is new and the EFI is in closed loop ( see 02 swings and
cross counts ) then we know all is well at idle and cruise.
The only way to prove it?, is with 4/5 gas analysis.